Legendary weapons and armor will have a max power limit that you can infuse the item to. For most items, this will be the max power for the season one year after the item was introduced. Raid gear has a higher max power limit. That's what is being referred to as sunsetting; the gear will still be available to use, but you won't be able to increase its power level beyond a certain point. This doesn't really affect PvP outside of Iron Banner and Trials of Osiris, as power level doesn't matter in all other playlists. It'll make more of an impact in PvE though, as newer content will have higher recommended power levels, so using a sunset weapon may bring your power down too much.